This invention relates to an improved barricade for use in protecting laboratory personnel and equipment from explosion hazards, and more particularly, to a lightweight structure, suitable for placement in a laboratory hood, which is designed to contain and vent the products of an accidental explosion while minimizing noise hazards.
Barricades are structures designed to prevent injury or damage from violent forces. Investigation of potentially explosive processes must be undertaken in a manner which reduces possible personnel injury and facility damage to a tolerable minimum. Traditionally, large and costly containment structures have been employed which were designed to house all parts of the potentially hazardous system, and which relied upon massive containment walls, remote sensing and control apparatus, and stringent safety procedures for their effectiveness.
Recent advances in miniaturization of test reactors and process systems have provided valuable savings in research and development costs. These microreactor systems employ milliliter quantities of reactants, concomitantly low energy inputs, small-scale conduits, pumps, mixing apparatus, and other specialized components. Refinements in instrumentation and analysis have permitted useful data to be obtained from microreactor systems, which can then be extrapolated to intermediate pilot plant and large scale production systems.
Despite the small scale of microreactor systems, a significant explosion hazard can be created depending upon the reaction or process being investigated. The magnitude of this potential explosion hazard may be expressed in terms of an equivalent force generated by an explosion of a given quantity of TNT (trinitrotoluene), or TNT equivalent (TNTE). On a scale relevant to the present invention, this potential hazard ranges between 0.3 and 100 g TNTE.
In order to realize the full cost and convenience advantages of microreactor technology, appropriately-scaled protective facilities are needed.
Large barricade and containment structure design has proceeded along two paths. The first approach opts for total containment of increased pressure, projected missiles, and noise resulting from an explosion. This "containment vessel" approach relies upon the configuration, massiveness, and strength of the vessel walls for its effectiveness. Experimentation undertaken in such a barricade must be carefully monitored to avoid exceeding strength limitations of the structure. One commercially available full containment vessel weighs approximately 1490 kg (4000 lbs.) and is designed to contain an explosive energy equivalent of 340.5 g (0.75 lb.) of TNT.
The second approach seeks to vent blast pressure and expanding gases of a contained explosion into the external environment of the barricade relatively instaneously. Certain designs retain only shrapnel or projectiles with mesh-like or net-like structures, permitting a pressure wave and expanding gas to escape the structure essentially unimpeded. Another variant of this approach employs "blowout ports" or vents, which may take the form of doors or aperture covers which are displaced by blast pressure, opening venting ports through which the expanding gases escape the containment vessel.
Since these "blowout port" systems vent explosion forces essentially instantaneously into the external environment, they are typically not located within conventional laboratory buildings. A blowout incident occurring in a conventional laboratory structure could be expected to result in personnel injury and facility damage due to shock and noise effects.
In regard to hazardous noise, the United States Occupational Safety and Health Administration (OSHA) has promulgated the following standard: "Exposure to impulsive or impact noise should not exceed 140 dB (decibels) peak sound pressure level." 29 C.F.R. 517 1910.95(a), (1981). For comparison purposes, Penninger and Okazaki, Chem. Eng. Prog., 76:6, p. 65, report that 144 dB represents an average human threshold of pain, and that 157 dB can be expected to result in glass window damage.
The design of containment vessels and explosion venting systems has, to a large extent, proceeded on an empirical basis. For example, Loving, U.S. Pat. No. 3,165,916, discloses a noise-reducing structure and a formula for calculating static pressures in total containment vessels, which can be employed to assist in selecting materials and configurations for containment vessels.
According to Loving, the static pressure acting upon the walls of a containment structure during a contained explosion may be estimated by the formula EQU P=K(W/V),
where P is static pressure (psi), K is a charge-dependent constant (2.times.10.sup.4 in the case of TNT), W is charge weight (lbs.), and V is containment structure volume (ft.sup.3). The Loving formula predicts a static pressure of 11,405 kPa (1,654 psi) for a detonation of 100 g TNTE within a structure of volume 75,710 cm.sup.3 (2.66 ft.sup.3).
Penninger and Okazaki, Chem. Eng. Prog. 76:6 pp. 65-71, disclose a containment barricade constructed in accordance with the Loving formula, designed to contain the explosive effects of 0.907 kg TNTE. This design incorporates a baffled vent duct.
Weibull, Ann. N.Y. Acad. Sci., v. 152 pp. 357-361, reports an empirically-derived formula for calculation of peak mean pressures in containment vessels. For contained TNT explosions within a charge weight/volume range of 0-5 kg/m.sup.3, Weibull predicts a peak mean pressure EQU p=22.5 (Q/V).sup.0.72,
where p is peak mean pressure in bars, Q is charge weight (kg), and V is chamber volume (m.sup.3). Weibull's formula predicts a peak mean pressure of 2,731 kPa (396 psi) as a result of a detonation of 100 g TNTE in a chamber of volume 75,710 cm.sup.3 (2.66 ft.sup.3). Weibull tested chambers with vent openings, but concluded that the vent openings of the test chambers, which varied within a contained volume/vent area range of 5080 cm (2000 in) to 27,208 cm (10,909 in) had "no noticeable influence" upon the peak mean pressures measured.